Recently, an electronic endoscope equipped with image pickup means has been widely used not only for conducting a normal observation but also for conducting the special light observation.
For example, Japanese Unexamined Patent Application publication No. 2002-95635 discloses the endoscope apparatus for conducting the special light observation to provide a narrow-band light observation image.
When the narrow-band light observation image is provided, the irradiation light intensity is reduced owing to the narrow-band. If the image information derived from the image pickup device is directly outputted to the monitor, the resultant image may become darker than the image obtained through the normal light observation.
As conventionally employed means for compensating the brightness, a light modulation signal is generated to control the opening/closing degree of the aperture of the light source device so as to adjust the illumination light intensity.
Conventionally, a signal picked up by the AGC circuit is amplified to an appropriate level. When the narrow-band light observation image is obtained, the irradiation light intensity is reduced owing to the narrow-band. Even if the illumination light intensity is maximized by controlling the aperture, there may be the case where the light intensity is still insufficient. In the aforementioned case, the signal has been electrically amplified by the AGC circuit and the like.
The dark image caused by the insufficient light intensity indicates the low S/N state. If the signal is amplified by the AGC circuit to realize the predetermined brightness, the noise becomes easily noticeable.
The method for smoothing in the frequency space has been known to suppress the noise as described above. For example, the image data are orthogonally transformed with Fourier base, which is further subjected to the inverse conversion after applying the low-pass frequency filter function. The similar effect may be realized by the process in the real space. Alternatively, the noise suppression method with a local filter has been well known.
In most of the aforementioned methods, the entire image is subjected to the process uniformly. This may reduce the contrast of the image information except noise, for example, image information with respect to the body tissue.
It is an object of the present invention to provide an image processor and an endoscope apparatus for providing an image suitable for the diagnosis by effectively suppressing the noise while alleviating lowering of the contrast.